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<div  id='write'  class = 'is-node'><p><span>Promise 对象用于表示一个异步操作的最终状态（完成或失败），以及其返回的值。</span></p><p><code>Promise</code><span> 对象是由关键字 </span><code>new</code><span> 及其构造函数来创建的。构造函数会，把一个叫做“处理器函数”（executor function）的函数作为它的参数。这个“处理器函数”接受两个函数</span><code>resolve</code><span> 和 </span><code>reject</code><span> 作为其参数。当异步任务顺利完成且返回结果值时，会调用 </span><code>resolve</code><span> 函数，而当异步任务失败且返回失败原因（通常是一个错误对象）时，会调用</span><code>reject</code><span> 函数。</span></p><p><strong><span>promise 状态</span></strong></p><p><span>　　　　pending：初始状态，既不是成功，也不是失败状态</span>
<span>　　　　fulfilled：操作成功</span>
<span>　　　　rejected：操作失败</span></p><h2><a name="promise-demo" class="md-header-anchor"></a><span>promise demo</span></h2><p><a href='javascript:void(0);'><img src="https://common.cnblogs.com/images/copycode.gif" referrerpolicy="no-referrer" alt="复制代码"></a></p><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 8.16406px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 1 &nbsp;  var promise1 = new Promise(function(resolve, reject) {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 2 &nbsp; &nbsp; &nbsp; &nbsp; setTimeout(function() {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 3 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; resolve('foo');</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 4 &nbsp; &nbsp; &nbsp; &nbsp; }, 300);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 5 &nbsp; &nbsp; });</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 6 &nbsp; &nbsp; promise1.then(function(value) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 7 &nbsp; &nbsp; &nbsp; &nbsp; console.log(value);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 8 &nbsp; &nbsp; &nbsp; &nbsp; // foo</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 9 &nbsp; &nbsp; });</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">10 &nbsp; &nbsp; console.log(promise1);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">11 &nbsp; &nbsp; //  [object Promise]</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 248px;"></div><div class="CodeMirror-gutters" style="display: none; height: 248px;"></div></div></div></pre><p><a href='javascript:void(0);'><img src="https://common.cnblogs.com/images/copycode.gif" referrerpolicy="no-referrer" alt="复制代码"></a></p><p><span>通过外部then() 方法来绑定成功、失败的回调函数，有没有感觉这个跟之前的ajax 差不多，只不过是我们把回调丢到了then() 中，这个then 并且支持链式操作，即如果存在多个嵌套那么也就是不断的then()。</span></p><h1><a name="async-await-字面理解" class="md-header-anchor"></a><span>async await 字面理解</span></h1><p><span>　　先从字面意思来理解，async 是“异步”的意思，而 await 是等待的意思。所以应该很好理解 async 用于申明一个 异步的function（实际上是async function 对象），而 await 用于等待一个异步任务执行完成的的结果。</span></p><p><span>并且 await 只能出现在 async 函数中。</span></p><h1><a name="asyncawait-如何执行" class="md-header-anchor"></a><span>async、await 如何执行</span></h1><p><span>async 告诉程序这是一个异步操作，await 是一个操作符，即 await 后面是一个表达式。</span></p><p><span>async 的返回值</span></p><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 8.16406px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">1  // async</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">2 &nbsp; &nbsp; async function testAsync() {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">3 &nbsp; &nbsp; &nbsp; &nbsp; return "hello async";</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">4 &nbsp; &nbsp; }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">5 &nbsp; &nbsp; const data = testAsync();</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">6 &nbsp; &nbsp; console.log(data);</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 135px;"></div><div class="CodeMirror-gutters" style="display: none; height: 135px;"></div></div></div></pre><p><span>如图所示：</span><img src="https://img2018.cnblogs.com/blog/825196/201809/825196-20180916175532787-1675892261.png" referrerpolicy="no-referrer" alt="img"></p><p><span>当调用一个 async 函数时，会返回一个 Promise 对象。根据mdn的解释</span></p><p><span>当这个 async 函数返回一个值时，Promise 的 resolve 方法会负责传递这个值；</span></p><p><span>当 async 函数抛出异常时，Promise 的 reject 方法也会传递这个异常值。async 函数中可能会有 await 表达式，await表达式会使 async 函数暂停执行，直到表达式中的 Promise 解析完成后继续     执行 async中await 后面的代码并返回解决结果。</span></p><p><span>注意， await 关键字仅仅在 async function中有效</span></p><p><span>既然返回的是Promise 对象，所以在最外层不能用 await 获取其返回值的情况下，那么肯定可以用原来的方式：then() 链来处理这个 Promise 对象 如</span></p><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 8.16406px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">1 &nbsp; &nbsp; // async</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">2 &nbsp; &nbsp; async function testAsync() {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">3 &nbsp; &nbsp; &nbsp; &nbsp; return "hello async";</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">4 &nbsp; &nbsp; }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">5 &nbsp; &nbsp; let data = testAsync().then( (data) =&gt; {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">6 &nbsp; &nbsp; &nbsp; &nbsp; console.log(data) // hello async &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">7 &nbsp; &nbsp; &nbsp; &nbsp; return data</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">8 &nbsp; &nbsp; });</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">9 &nbsp; &nbsp; console.log(data);</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 225px;"></div><div class="CodeMirror-gutters" style="display: none; height: 225px;"></div></div></div></pre><p><span>如果 async 函数没有返回值，又怎么样呢？很容易想到，它会返回 Promise.resolve(undefined)。</span></p><p><span>联想一下 Promise 的特点无等待，所以在没有 await 的情况下执行 async 函数，它会立即执行，返回一个 Promise 对象，并且，绝不会阻塞后面的语句。</span></p><h1><a name="await-操作符" class="md-header-anchor"></a><span>await 操作符</span></h1><p><span>MDN 是这样描述 await 的：</span></p><p><span>await 表达式会暂停当前 async function 的执行，等待 Promise 处理完成。若 Promise 正常处理(fulfilled)，其回调的resolve函数参数作为 await 表达式的值，继续执行async function。若 Promise 处理异常(rejected)，await 表达式会把 Promise 的异常原因抛出。另外，如果 await 操作符后的表达式的值不是一个 Promise，则返回该值本身。</span></p><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 8.16406px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">async 函数返回一个 Promise 对象，当函数执行的时候，一旦遇到 await 就会先返回，等到触发的异步操作完成，再接着执行函数体内后面的语句。</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 45px;"></div><div class="CodeMirror-gutters" style="display: none; height: 45px;"></div></div></div></pre><p><span>按照mdn解释 await会暂停当前async 函数执行，并且await 后面是一个表达式，即这个await 等待的是一个表达式（这个表达式返回promise 对象或者一个具体的值）：</span></p><ul><li><span>假如这个表达式如果返回的是一个Promise 对象， 那么它的返回值，实际上就是 Promise 的回调函数 resolve 的参数，如果这个Promise rejected 了，await 表达式会把 Promise 的异常抛出。</span></li><li><span>假如这个表达式如果返回的是一个常量，那么会把这个常量转为Promise.resolve(xx)，同理如果没有返回值也是Promise.resolve(underfind)</span></li></ul><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 8.16406px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">1 async function testAwait() {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">2 &nbsp; &nbsp; &nbsp; &nbsp;  const data = await "hello await";</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">3 &nbsp; &nbsp; &nbsp; &nbsp;  console.log(data) &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">4 &nbsp; &nbsp; &nbsp; &nbsp; return data</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">5 }</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 113px;"></div><div class="CodeMirror-gutters" style="display: none; height: 113px;"></div></div></div></pre><p><span>输出 “hello await”</span></p><p><strong><span>返回promose 对象，成功状态</span></strong></p><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="" style="break-inside: unset;"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 8.16406px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 1 &nbsp; &nbsp; function say() {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 2 &nbsp; &nbsp; &nbsp; &nbsp; return new Promise(function(resolve, reject) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 3 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; setTimeout(function() {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 4 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; let age = 26</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 5 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; resolve(`hello, joel。今年我 ${age} 岁`);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 6 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }, 1000);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 7 &nbsp; &nbsp; &nbsp; &nbsp; });</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 8 &nbsp; &nbsp; }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 9 </span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">10 &nbsp; &nbsp; async function demo() {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">11 &nbsp; &nbsp; &nbsp; &nbsp; const v = await say(); // 输出：hello, joel。今年我 26 岁  等待这个say 的异步，如果成功把回调 resole 函数的参数作为结果</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">12 &nbsp; &nbsp; &nbsp; &nbsp; console.log(v);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">13 &nbsp; &nbsp; }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">14 &nbsp; &nbsp; demo();</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 338px;"></div><div class="CodeMirror-gutters" style="display: none; height: 338px;"></div></div></div></pre><p><strong><span>返回promise 对象，失败状态</span></strong></p><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="" style="break-inside: unset;"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 8.16406px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 1 function say() {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 2 &nbsp; &nbsp; &nbsp; &nbsp; return new Promise(function(resolve, reject) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 3 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; setTimeout(function() {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 4 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; let age = 26</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 5 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; reject(`hello, joel，发生了异常。今年我 ${age} 岁`);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 6 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }, 1000);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 7 &nbsp; &nbsp; &nbsp; &nbsp; });</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 8 &nbsp; &nbsp; }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 9 &nbsp; &nbsp; async function demo() {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">10 &nbsp; &nbsp; &nbsp; &nbsp; try {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">11 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; const v = await say(); // 输出：hello, joel，发生了异常。今年我 26 岁  等待这个say 的异步，如果成功把回调 resole 函数的参数作为结果</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">12 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; console.log(v);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">13 &nbsp; &nbsp; &nbsp; &nbsp; } catch (e) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">14 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; console.log(e)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">15 &nbsp; &nbsp; &nbsp; &nbsp; }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">16 &nbsp; &nbsp; }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">17 &nbsp; &nbsp; demo();</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 405px;"></div><div class="CodeMirror-gutters" style="display: none; height: 405px;"></div></div></div></pre><p><span> async/await 相比原来的Promise的优势在于处理 then 链，不必把回调嵌套在then中，只要await 即可，如</span></p><pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="" style="break-inside: unset;"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 0px; left: 8.16406px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 1 &nbsp; &nbsp; function sing() {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 2 &nbsp; &nbsp; &nbsp; &nbsp; return new Promise(function(resolve, reject) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 3 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; setTimeout(function() {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 4 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; resolve(`来一首好听的歌吧~~~`);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 5 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }, 1000);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 6 &nbsp; &nbsp; &nbsp; &nbsp; });</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 7 &nbsp; &nbsp; }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 8 &nbsp; &nbsp; async function demo() {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> 9 &nbsp; &nbsp; &nbsp; &nbsp; try {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">10 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; const v = await say(); </span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">11 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; const s = await sing(); </span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">12 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; console.log(v); // 输出：hello, joel。今年我 26 岁</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">13 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; console.log(s) // 来一首好听的歌吧~~~</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">14 &nbsp; &nbsp; &nbsp; &nbsp; } catch (e) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">15 &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; console.log(e)</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">16 &nbsp; &nbsp; &nbsp; &nbsp; }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">17 &nbsp; &nbsp; }</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">18 &nbsp; &nbsp; demo();</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 405px;"></div><div class="CodeMirror-gutters" style="display: none; height: 405px;"></div></div></div></pre><p><span>如果使用原来的Promise 就是把回调放在then()中。</span></p><h1><a name="总结" class="md-header-anchor"></a><span>总结</span></h1><ol><li><span>async 告诉程序这是一个异步，awiat 会暂停执行async中的代码，等待await 表达式后面的结果，跳过async 函数，继续执行后面代码</span></li><li><span>async 函数会返回一个Promise 对象，那么当 async 函数返回一个值时，Promise 的 resolve 方法会负责传递这个值；当 async 函数抛出异常时，Promise 的 reject 方法也会传递这个异常值</span></li><li><span>await 操作符用于等待一个Promise 对象，并且返回 Promise 对象的处理结果（成功把resolve 函数参数作为await 表达式的值），如果等待的不是 Promise 对象，则用 Promise.resolve(xx) 转化</span></li></ol></div>
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